The invention generally relates to systems and methods for providing logarithmic analog-to-digital conversion, and relates in particular to circuits for providing logarithmic analog-to-digital conversion for auditory enhancement.
Cochlear implants (or bionic ears) have been implanted in tens of thousands of people worldwide. Cochlear implants use a surgically implanted array of electrodes to stimulate the auditory nerve, which restores a measure of hearing to the deaf. The cochlear implant functions as a surrogate middle and inner ear, performing the same chain of signal transduction from sound waves to electrical impulses as in the biological cochlea. FIG. 1 shows the signal-processing chain of a typical cochlear implant that includes a preamplifier 10 that receives an input signal via a microphone 12. The preamplifier 10 provides an amplifier output signal to a plurality of band pass filters 14a–14c. The outputs of the band pass filters 14a–14c are provided to a plurality of envelope detectors 16a–16c respectively, and the outputs of the envelope detectors 16a–16c are provided to a plurality of logarithmic map units 18a–18c respectively. The outputs of the logarithmic map units 18a–18c are provided to a plurality of electrodes 20a–20c respectively as shown. During operation, each band pass filter 14a–14c passes a different band (e.g., low, mid, high). The envelope detectors are used to identify envelope information, and the logarithmic map units are used to provide logarithmic compression of the signal, which is conventionally employed just prior to nervous stimulation. Such compression is useful because theory and experiments suggest that acoustic amplitudes are log-compressed into electrical amplitudes by the cochlea. In other words, there is believed to be a linear relationship between sound intensity in dB sound pressure level (SPL) and electrical stimulation intensity in μA. A circuit that performs logarithmic compression, therefore, is useful in cochlear implant processors.
Logarithmic compression is also inherent to cepstral speech recognition, since a logarithmic function allows the excitation signal in speech to be subtracted from the effect of filtering by the vocal tract. Many speech-recognition front-ends are therefore designed to produce output bits that represent log-spectral magnitudes of a microphone input signal. A logarithmic A/D logarithmically compresses the current input from an envelope detector into a digital output code. Consequently, a low-power logarithmic A/D is very useful in portable speech-recognition front-ends and bionic ears.
Conventional cochlear implants and speech-recognition front-ends typically employ an analog-to-digital (A/D) conversion, followed by a digital signal processor (DSP) to perform the necessary signal processing. A DSP, however, is costly in power when compared with analog processing schemes.
There is a need for a logarithmic A/D system that provides significant power savings for fully-implanted cochlear implants or speech-recognition-front-ends. There is further a need for a logarithmic A/D (logmap) that exploits sub-threshold CMOS technology to compute a logarithm in a fraction of the power of DSP implementations.